1. Field of Invention
The present invention relates in general to electrophotography and, in particular, to a process for preparing electrophotographic imaging members or photoreceptors. The present invention provides a process for forming such imaging members, and imaging members formed thereby, having improved adhesion between coated layers.
2. Description of Related Art
In electrophotography, also known as Xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. In addition, the imaging member may be layered. Current layered organic imaging members generally have at least a substrate layer and two active layers. These active layers generally include (1) a charge generating layer containing a light-absorbing material, and (2) a charge transport layer containing electron donor molecules. These layers can be in any order, and sometimes can be combined in a single or mixed layer. The substrate layer may be formed from a conductive material. In addition, a conductive layer can be formed on a nonconductive substrate.
The charge generating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer. For example, U.S. Pat. No. 4,855,203 to Miyaka teaches charge generating layers comprising a resin dispersed pigment. Suitable pigments include photoconductive zinc oxide or cadmium sulfide and organic pigments such as phthalocyanine type pigment, a polycyclic quinone type pigment, a perylene pigment, an azo type pigment and a quinacridone type pigment. Imaging members with perylene charge generating pigments, particularly benzimidazole perylene, show superior performance with extended life.
In the charge transport layer, the electron donor molecules may be in a polymer binder. In this case, the electron donor molecules provide hole or charge transport properties, while the electrically inactive polymer binder provides mechanical properties. Alternatively, the charge transport layer can be made from a charge transporting polymer such as poly(N-vinylcarbazole), polysilylene or polyether carbonate, wherein the charge transport properties are incorporated into the mechanically strong polymer.
Imaging members may also include a charge blocking layer and/or an adhesive layer between the charge generating and the conductive layer. In addition, imaging members may contain protective overcoatings. Further, imaging members may include layers to provide special functions such as incoherent reflection of laser light, dot patterns and/or pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface.
Suitable coating methods used for applying the various layers in electrophotographic imaging members include dip coating, roll coating, Meyer bar coating, bead coating, curtain flow coating and vacuum deposition. Solution coating is a preferred approach because it is more economical than vacuum coating and can be used to deposit a seamless layer.
U.S. Pat. No. 4,855,203 to Miyaka teaches applying charge generating layers from coating solutions comprising a resin dispersed pigment. Miyaka discloses suitable organic solvents for preparing a coating solution of the pigments as including alcohols such as methanol, ethanol and isopropanol; ketones such as acetone, methylethyl ketone and cyclohexanone; amides such as N,N-dimethyl formamide and N,N-dimethyl acetamide; sulfoxides such as dimethyl sulfoxide; ethers such as tetrahydrofuran, dioxane and ethylene glycol monomethyl ether; esters such as methyl acetate and ethyl acetate; aliphatic halogen hydrocarbons such as chloroform, methylene chloride, dichloroethylene, carbon tetrachloride and trichloroethylene; or aromatic compounds such as benzene, toluene, xylene, ligroin, monochlorobenzene and dichlorobenzene.
U.S. Pat. No. 3,904,47 to Regensburger et al. teaches applying perylene containing charge generating layers by a vacuum coating process. Vacuum coated charge generating layers containing perylenes show a high photosensitivity. However, vacuum coating is expensive.
U.S. Pat. No. 5,521,047 to Yuh et al. is directed to a process for preparing an electrophotographic imaging member having a perylene-containing charge generating layer from solution. The process comprises forming a dispersion of a perylene pigment and a polyvinylbutyryl binder in an acetate solvent and applying the dispersion to an electrophotographic imaging member layer by solution coating. Yuh et al. teaches that perylenes form stable dispersions in acetate solvents for the purposes of application by solvent coating such as dip coating.
U.S. Pat. No. 5,891,594 to Yuh et al. discloses a process for preparing an electrophotographic imaging member having a perylene-containing charge generating layer. The process includes the steps of dispersing a perylene-containing charge generating material in a solvent comprising n-butylacetate and a second solvent having a lower boiling point than n-butylacetate, wherein the second solvent is an acetate or tetrahydrofuran, and applying the dispersion to form the charge generating layer on a substrate or underlayer of the imaging member.
As described in the above-cited patents, solution coating is a more economical and convenient method of applying charge generating and charge transport layers than other of the known application methods. However, solution coating poses several problems that need to be overcome. For example, in the case of some particular charge generating materials such as perylene pigments, it may be difficult to disperse the materials in a coating solution, and unstable dispersions may be encountered when coating the materials from solution. Such unstable dispersions can cause pigment flocculating and settling that leads to coating quality problems. In addition, unstable dispersions are difficult to process, especially in a dip coating process. Further, some dip coated materials show a substantial depreciation in photosensitivity as compared to otherwise less preferred vacuum coated layers.
Furthermore, it is desired in the art to increase the adhesion between successive layers in an imaging member package. In particular, in the case of endless (seamless) belts, which tend to undergo much mechanical stress, increased adhesion of the successive layers in the imaging member is particularly desired.
Another problem with dip coating processes is that in some instances, the concentration of material to be coated can not be maintained at a desired level. For example, U.S. Pat. No. 5,709,974 discloses that, in the case of aromatic diamine charge transport coating materials, the maximum concentration of the aromatic diamine that can be dispersed in a binder is limited in a dip coating process due to the long residence time of the solvent before the drying step occurs. Thus, phase separation of the diamine can occur during the solvent resident time. Phase separation is undesirable because phase separation can result in poor charge transport including residual build, which adversely affects print quality.